GeezerVison

Problem: Packer.pack_map_pairs() retains partial output when a serializer callback raises, so the next successful call can return corrupted data.

Fix: Clear the buffer after packing errors in both the C extension and Python fallback, matching Packer.pack(). Preserve the original exception.

Validation: 147 native tests and 144 fallback tests passed. Regression coverage checks both autoreset modes, recovery, and successful buffered streams. Ruff, formatting, and package builds passed.